This invention relates to high voltage power cables, more particularly, to a high voltage electric power cable including a superconducting conductor, and a cable dielectric which operates at room temperature.
A power transmission system typically includes a generator located in a remote area, a transformer to raise the voltage and lower the current output of the generator, a transmission line or cable to transfer the power to a load which is located in a developed area, a substation to receive the transmitted power and transform it from high voltage to a lower distribution voltage and send it out over many distribution lines to customers. The power transmitted is the product of the current and voltage. Alternating current (AC) transmission is characterized by voltages of 69 kV to 765 kV while direct current (DC) transmission is generally in the range of 100 kV to 600 kV.
Large amounts of power may be transmitted using either an overhead line or an underground cable. An overhead line is typically a bare conductor using the ambient air as its dielectric, while an underground cable typically has one or more conductors surrounded by a dielectric which is in turn surrounded by a ground shield that provides a well defined electric field configuration, thereby ensuring the power arrives at its destination. An underground cable is about ten times more expensive to install than an overhead line, so underground cables are used only where community or aesthetic considerations render overhead lines inappropriate. A typical use for high voltage underground cables is as a link between the terminus of overhead lines from a remote generator at the edge of a city and a substation serving the city.
Most transmission cable systems in North America include three insulated conductor assemblies within a steel pipe which contains a dielectric fluid, such as mineral oil or synthetic oil, for example, Dichevral.TM. 100, pressurized to approximately 200 psig. Each conductor assembly includes a large conductor, such as copper or aluminum, having a cross section in the range of 500 mm.sup.2 to 2000 mm.sup.2, which is surrounded by carbon-impregnated paper to provide a smooth electrical transition to a dielectric consisting of dielectric fluid impregnated paper or paper polypropylene laminate. The dielectric is further surrounded by carbon impregnated paper and a conductive shield, such as a taped metallic shield, at ground potential. Skid wires with D-shaped cross section are wound in a spiral around the ground shield to protect a length of cable, typically 0.25 to 1 km, while it is pulled in the steel pipe.
Cable conductors, such as copper and aluminum, have electrical resistance, and cable dielectrics have dielectric losses, all resulting in substantial amounts of heat being generated per unit length of cable. The laminar or extruded dielectric between the cable conductor and the conductive ground shield has limited ability to operate at high temperatures. The ultimate limit on the power transfer of a power cable system is normally the maximum allowable temperature of the cable dielectric, above which the operating life of the cable degrades rapidly. A power cable system is designed so that at its rated power, the heat generated by the cable can be dissipated safely into the soil in which the cable is buried without exceeding the maximum allowable temperature for the cable dielectric. One or more cables are typically located inside a pipe which is in a deep trench backfilled with a mixture selected for its high thermal conductivity, such as certain compacted crushed stones or sand mixtures.
Approximately sixty percent of the cost of an initial installation of an underground cable system is due to the costs of installing the cable pipe in the ground, including trenching, handling and installing of the pipe, and backfilling the trench with thermal backfill.
Many communities are now served by underground power cables that have been in place for decades. Growth in power usage requires increasing the amount of power delivered to such communities. It is highly desirable to retrofit the existing power cable pipes to provide increased capacity, rather than installing new pipes, so as to avoid the majority of the cost associated with the cable system, namely, the costs of installing new cable pipe. If existing pipes are retrofitted, the new cable installed in the pipes must, of course, fit within the diameter of the existing pipes. It is also desirable that the finished assembly be similar in nature to the conventional cable assembly so as to enjoy the benefit of proven experience.
The type of cable used in a cable pipe retrofit depends on the amount and type of increase in power transmission capacity which is desired. If the capacity is to be increased by increasing the voltage levels, conventional non-superconducting cables are sometimes adequate due to improvements in power cable technology during the last few decades. However, if capacity is to be increased by increasing the current carrying capability, a problem arises with the use of conventional cables, namely, existing cable pipe is too small to dissipate the heat generated from a non-superconducting cable having sufficient power transmission capacity. Thus, it is necessary to use a superconducting cable.
In a superconducting cable, the metallic conductor of a conventional cable is replaced with a superconductor that can carry a larger current with a lower loss, in the case of AC transmission, or zero resistive loss in the case of DC transmission. However, the superconductor must be maintained at a temperature low enough to remain superconductive. A cryogen such as liquid helium or liquid nitrogen is a cold material which is able to maintain a superconductor at a sufficiently low temperature for superconduction to occur. The cryogen and superconductor are enclosed within a cryostat which provides thermal insulation from the surrounding environment. Liquid helium at a temperature of 4-10 Kelvin (K) is used for low temperature superconductors (LTSCs), while liquid nitrogen at a temperature of 68-85 K is used for high temperature superconductors (HTSCs). The surrounding environment is considered to be at a room temperature of 300 K.
Superconducting cable designs having only one superconductor suffer losses due to eddy currents and circulating currents induced in the ground shield by the magnetic field resulting from the high current and also by interaction with magnetic fields created by other superconductors that are in close proximity such as in a three-phase transmission system. LTSCs which operate in liquid helium incur a severe cost penalty because these losses must be removed by the cryogen.
This problem was solved by using a superconductor for the outer shield of the cable so as to confine the magnetic field to the space between the superconductors and eliminate the driving force for eddy currents and circulating currents in the outer metallic parts of the cable. Both superconductors had to be maintained at low temperatures. Consequently, the dielectric between the superconductors also had to function at the low temperature, that is, a cryogenic dielectric was required. Cost and space considerations resulted in the placement of the coaxial superconductors and the cryogenic dielectric therebetween in one large cryostat.
A liquid helium impregnated dielectric was maintained in a supercritical state so as to avoid the formation of bubbles of gas, caused by boiling, which have a lower dielectric breakdown strength than liquids.
HTSC cable is much less costly than LTSC cable, mainly due to savings in refrigeration costs. For instance, to remove one watt of heat from a superconducting cable requires about 250-500 watts of refrigeration power for a LTSC cable but only about 10-15 watts of refrigeration power for a HTSC cable.
Conventional LTSC designs had to be coaxial, and thus, had to use a cryogenic dielectric to reduce losses to an absolute minimum because of the high energy costs of removing heat caused by losses.
The cryostat used in LTSC designs is large, cumbersome and relatively difficult, thus relatively costly, to retrofit into existing pipes. A cable with a flexible cryostat surrounding three coaxial cables and utilizing the maximum available space within an existing pipe results in very large assemblies that cannot practically be transported in long lengths comparable to conventional practice.
Materials suitable for operation at low temperatures as a dielectric required for the implementation of a coaxial design have not been successfully demonstrated. Therefore, the introduction of a superconducting cable system requiring a cryogenic dielectric is not practical at this time and awaits establishment of the viability of such a cryogenic dielectric.
Thus, there is a need for a superconducting cable that can operate with a well established dielectric, although such a cable might not be optimally efficient.